This invention relates to a control system for a vehicle safety device such as an air bag.
For example, as disclosed in U.S. Pat. No. 4,853,623 and Japanese Laid-Open Utility Model Application No. 2-5371, a control system for an air bag comprises an acceleration sensor, a signal processing circuit, such as a microcomputer, for processing a signal from the acceleration sensor, and a drive circuit for driving the air bag.
The signal outputted from the acceleration sensor represents one of voltages higher and lower than a reference voltage during the acceleration of a vehicle, and represents the other voltage during the deceleration of the vehicle. The microcomputer inputs and integrates the signal from the acceleration sensor at a predetermined cycle, and compares this integral value with a threshold level. This integral value represents a change in the speed of the vehicle, and increases in the decelerating direction upon collision of the vehicle. When the integral value exceeds the threshold level, the microcomputer judges that a collision has occurred, and outputs a trigger signal to the drive circuit to expand the air bag.
During the repair of the vehicle in a fixed condition, when an impact is applied to the vehicle by a hammer, a sensor signal of a very large amplitude is produced alternately in accelerating and decelerating directions. The acceleration and the deceleration at this time are generally equal in magnitude to each other. Therefore, when this hammer blow is applied, the integral value of the signal of the acceleration sensor calculated by the microcomputer must be kept generally at zero, and hence the air bag must be kept in an unexpanded condition. However, there remains the possibility of erroneous expansion of the air bag for the following reason. The microcomputer effects the sampling of the sensor signal at predetermined time intervals, and therefore there is a possibility that the sampling of the sensor signal is consecutively effected by accident when the deceleration is large whereas the acceleration is small. In this case, since the absolute value of the signal is very large, the integral value of the sensor signal increases in the decelerating direction, and exceeds the threshold level, thereby inviting an erroneous expansion of the air bag.
In order to reduce the possibility of erroneous expansion of the air bag when the above hammer blow is applied, the cycle of the sampling of the sensor signal may be shortened so as to increase the precision of the integral calculation. However, there is a limitation on this method. When the threshold level is set to a higher level in order to prevent the erroneous expansion of the air bag, it takes long time for the integral value of the deceleration to reach the threshold level when an actual collision occurs, so that the expansion of the air bag is delayed.